Wheel revolution counters typically comprise a substantially cylindrical, metallic housing, means at one end of the housing for a cantilevering or overhanging fastening of the wheel revolution counter at a wheel hub of a motor vehicle, and at least one contact part electrically insulated relative to the housing and fastened at the housing's other end. When the distances covered by motor vehicles, trailers or semi-trailers are to be determined in an extensively tamper-proof manner, e.g., because the kilometers traveled by these vehicles are to be taxed, it is conventional to use such wheel revolution counters as distance counters. The latter are usually attached directly to the wheel hub of the respective vehicle and accordingly require neither flexible shafts nor electrical cables for the transmission of the measured values. Since distance counters of this type revolve with the vehicle wheel to which they are assigned, it is necessary for the counting process to provide a zero comparison rate of revolutions, which is usually realized by means of a pendulum supported in the wheel revolution counter.
In order to be able to assign the current kilometer status to a determined vehicle, which is compulsory in the event that distances are to be taxed, the wheel revolution counter must also supply identification information in addition to the kilometer status. This data is, of course, also useful for checking maintenance intervals, for monitoring wear, making transport calculations, or for determining the running performance of rented vehicles.
In DE C 38 41 509, a wheel revolution counter is suggested which, in contrast to known printing wheel revolution counters, is constructed as a hermetically closed plug-in connection part from which the data can be extracted via contacts. The contacts are preferably constructed on the front side and in a planar manner at this wheel revolution counter, which comprises a substantially cylindrical housing, so that there is no construction space requirement inside the device and the plug-in connection is independent of the penetration of dirt; in other words, the inevitable dirt in the environment of use can be removed. The plug-in connection, via which the data exchange is effected, is further constructed with at least one self-enclosed contact path in such a way that a position-independent coupling of an autonomous data transfer device, or a "plug" connected with a cable, or a sensing or scanning head of a data transfer device, is provided.
The advantage of this electrical coupling consists in that the energy for reading out the data can be supplied externally to the wheel revolution counter. In so doing, a minimum of contacts can be achieved by means of selecting a suitable data transmission process, namely, a serial data transmission process, in connection with a suitable data transmission control. In contrast, optical and inductive or capacitive transmission means are relatively unsuited for such use due to the narrow temperature range and the film of dirt, which cannot be entirely eliminated in this environment, or due to interference by electrical and magnetic fields, as well as the higher cost of circuitry.
In the electrical coupling, the insulation of the contacts and the tightness of the contact connections or contact bridges leading into the device are particularly a problem in the environment of use of a wheel revolution counter at the axle of a vehicle, where blows from stones and aggressive media are common and corrosion-resistant steel must accordingly preferably be used for the housing of the wheel revolution counter, and where there are extreme temperature differences, freezing, moisture and electrically conductive dirt. This means that great demands are to be imposed on the contact sealing means with respect to resistance to temperature and aging, and the sealing locations are to be extensively reduced and optimized for a secure sealing effect. On the other hand, the manufacturing conditions ranging from mass production to mechanical assembly must be taken into account in the development of such a device.